Many of you over about 45 years of age will recognise the object below as a British Thornton slide rule - the tool we used in school immediately prior to the introduction of the Casio electronic calculator to do advanced maths. Calculus, logs, cosines and the rest were figured by lining up points on the sliding centre bar and reading the answering figure from the cursor hairline. You can buy them on eBay for under a tenner. No batteries, no electronics and therefore immune from EM radiation, showerproof and therefore a useful aid in the boatyard, it earns its place alongside the kerosene pressure lanterns, the Cat 'C' first aid kit and the British Berkefeld water filter in the land grab-bag.
Except you may also need your reading glasses or a small magnifier these days to see the tiny figures. Grrr.